1. Field of the Invention
This invention relates to the field of devices for the protection of power electronic devices from extreme voltage or current changes and, in particular, to snubbers which protect switching devices in unipolar brushless DC motors and also return energy stored in such snubbers to the motors protected.
2. Description of the Related Art
The use of turn on and turn off snubbers for the protection of power electronic devices is not new. Snubbers in general are devices which limit overvoltage or overcurrent transients over electronic switching devices during or after the action of switching current. Snubbers have been used to protect devices against rapid changes in voltage with respect to time (dv/dt), rapid changes in current with respect to time (di/dt) and transient voltages.
Although dissipative snubbers are very common, regenerative snubbers have been proposed to return energy stored in snubber elements back to the positive voltage supply, thereby increasing system efficiency. Regenerative snubbers may be comprised of passive or active components. Regenerative snubbers using transformers have been proposed. An actively controlled regenerative snubber configuration for use in unipolar configuration brushless direct current (DC) motors is disclosed herein. The regenerative snubber is used to maintain a constant voltage across the switching devices to prevent braking of the motor through conduction of motor back electromotive force (EMF), and to return excess energy stored in the motor phase coils to the positive voltage supply. The return of energy to the positive rail is done in a manner so as to minimize conducted electromagnetic interference at the power leads.
The most common snubber configuration is the Resistor-Capacitor-Diode (RCD) snubber. This snubber configuration appears in FIG. 1. The principle of operation of the RCD snubber follows. When switch S2 is open, the current stored in L4 is discharged into the snubber capacitor C6 through diode D8. When switch S2 is closed again, the energy stored in C6 discharges through the resistor R10 back into the switch S2. As a result, the energy stored in the coil 4 is transferred to the capacitor, C6 and through the resistor R10. The disadvantage of the RCD snubber is the dissipation of stored energy in the form of heat. If the switching frequency of the device to which the snubber is attached is high, the amount of energy converted to heat may be excessive.
When using the RCD snubber in the control circuit of a unipolar brushless DC motor, there arise some unique challenges particularly where low voltage, high current applications are concerned. (In a unipolar brushless DC motor the current in the windings flows in one direction through the coils from a DC source to ground.) In such a system, the snubber capacitor and resistor are shared by the phases feeding the snubber network through a series of diodes. The typical RCD snubber configuration for unipolar brushless DC motors is illustrated in FIG. 2. The snubber resistor 12 is connected to the positive voltage rail 14 so that some of the energy stored in the snubber capacitor may be returned to the positive voltage rail.
In the low side drive configuration shown in FIG. 2, each particular phase has its own diode to feed the snubber network. The diodes are present to prevent the shorting out of the motor phase coils. (The low side drive configuration is called by this name since the switch S15 is connected to the low side or bottom side of the load. The low side drive configuration is also called a boost converter since V.sub.A 13 is always greater than V.sub.S 17 when the switch S15 is turned on and off rapidly.) Such a snubber has been proposed by Elliot et al., U.S. Pat. No. 4,678,973. The performance of the snubber resistor 12 is not optimized for all motor speeds. Therefore, at high motor speeds when there is no chopping, the snubber resistor acts as a brake. Consequently, the efficiency of the motor is adversely affected at high motor speeds. For a 600 watt (W) application, there would be an increase in losses and, therefore, the snubber circuit would dissipate more power.
An RCD snubber network is only required for low side drive, or boost converter type motor drives. In a high side drive, or buck converter motor drive, the RCD snubber may be replaced by diodes connected to the positive rail. This configuration is illustrated in FIG. 3. This configuration is so named since the switch S1 19 is connected to the high side or top side of the load. It is also called a "buck" converter or a "down" converter as V.sub.A 21 is never greater than V.sub.S 23. In some automotive systems, however, there is a requirement for reverse voltage protection.
In the high side drive configuration illustrated in FIG. 3, if the switching devices used are MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) which are best suited for high power, low voltage operation (low on resistance), the reverse protection devices must be included due to the source to drain diodes inherent to the MOSFETs. These diodes will become forward biased during a reverse polarity condition. The additional reverse protection devices have a voltage drop associated with them, causing a loss in motor efficiency.
The low side drive configuration, on the other hand, requires reverse voltage protection devices. However, in this configuration, only two voltage drops are dealt with, as opposed to three in the case of the high side drive, or buck converter, configuration.
In the boost, or low side drive configuration, passive means of energy recovery through the use of catch windings has been proposed by Finney et al., The RCD Snubber Revisited. IEEE Industrial Applications Society Conference Proceedings, Toronto, Canada, 1993, pp. 1267-1273. The disadvantage of this method is the size of the transformers needed to return the energy stored in the snubber capacitor back to the positive voltage supply. The transformer may be another phase coil, or part of the same winding as the phase coil. There is a cost penalty associated with the manufacture of bifilar windings (two windings per phase), however, in addition to the difficulty in winding them.
Active snubber configurations have been proposed by Zach et al., New Lossless Turn-On and Turn-Off (Snubber) Networks for Inventors, Including Circuits for Blocking Voltage Limitation, IEEE Transactions on Power Electronics, April 1986, pp. 65-75, and by Elasser and Torrey, Soft Switching Active Snubbers for DC/DC Converters. IEEE Applied Power Electronics Conference, Dallas, Tex., 1995, pp. 483-489. These snubber configurations use a transistor and an inductor to replace the snubber resistor. The purpose of the transistor and inductor is to discharge the snubber capacitor to the positive voltage rail, recycling the capacitor energy and thereby increasing system efficiency.
In unipolar brushless DC motors, the active snubber configuration may also be used to increase motor efficiency at all speeds. The added challenges with motors of this type involve dealing with motor back electromotive force (EMF), and the return of the capacitor energy to the positive rail in such a manner as to minimize conducted radio frequency noise emissions.